The present invention relates to electroluminescent driving apparatus and, more particularly, to an electroluminescent driving apparatus capable of controlling the luminance of light emitted from an electroluminescent element based on ambient brightness.
An electroluminescent (hereinafter referred to as EL) element is among conventional elements used in displays of electric-field light-emission type. For example, Japanese Laid-Open Patent Publication HEI 8-45661 has disclosed an EL driving apparatus for driving the EL element.
Referring to the drawings, the conventional EL driving apparatus disclosed therein will be described.
FIG. 16 shows a circuit configuration of the conventional EL driving apparatus. As shown in FIG. 16, the EL driving apparatus consists of a display portion 200, a photodetector portion 250, and a control unit 260.
The display portion 200 has: a series circuit composed of a coil 201 and a diode 202 connected in series at a common connection point; a first switch element 203 having a gate connected to the control unit 260, a drain connected to the common connection point, and a source connected to the ground; a power source 204 connected to the terminal of the series circuit closer to the coil 201; a second switch element 205 having a gate connected to the control unit 260, a drain connected to the terminal of the series circuit closer to the diode 202, and a source connected to the ground; and an EL element 206 connected in parallel with the series circuit between the power source 204 and the second switch element 205.
The photodetector portion 250 has: a photodiode 251 as a photodetector element for sensing ambient brightness; an operational amplifier 252 for receiving an output signal from the photodiode 252, amplifying the output signal, and outputting the amplified signal; an A/D converter 253 for receiving the amplified signal, converting the amplified signal to a digital signal, and outputting the digital signal; first and second resistors 254 and 255 connected in series at a common connection point to generate a specified voltage by resistive division; and a third switch 256 having a gate connected to the control unit 260, a drain connected to the terminal of the second resistor 255 on the opposite side of the common connection point, and a source connected to the ground. The terminal of the first resistor 254 on the opposite side of the connection point is connected to the power source 204 of the display portion 200.
The operational amplifier 252 has a positive-phase input terminal connected to the common connection point for the first and second resistors 254 and 255 and to the negative electrode of the photodiode 251 and a negative-phase input terminal connected to the positive electrode of the photodiode 251. The output signal from the operational amplifier 252 is feedbacked thereto via a third resistor 257.
The arrows accompanying the signs of the N-type MOS transistors indicative of the respective switch elements 203, 205, and 256 represent the sources thereof, which similarly applies to the other drawings.
A description will be given to the outline of the operation of the EL driving apparatus thus constituted.
On receiving a pulse signal C1 from the control unit 260, the display portion 200 repeats a sequence of operations of shifting the first switch element 203 from the OFF state to the ON state and returning the first switch element 203 to the OFF state and thereby accumulates charges in the electrode of the EL element 206 closer to the diode 202 by using a counter electromotive force generated at the coil 201, resulting in an increased voltage at the electrode. When the second switch element 205 is in the ON state because of a pulse signal C2 having a period longer than the period of the pulse signal C1 from the control unit 260, the display portion 200 releases the charges accumulated in the electrode of the EL element 206 closer to the diode 202 and thereby causes the EL element 206 to emit light.
The photodiode 251 of the photodetector portion 250 senses ambient brightness and a signal representing the sensed brightness is amplified by the operational amplifier 252 and converted to a digital signal by the A/D converter 253.
The control unit 260 receives the digital signal after conversion and changes the period during which the first switch element 203 is in the ON state based on the digital signal. Specifically, the ON-state period is elongated when the ambient brightness is high to increase the value of the increased voltage applied to the EL element 206 or the ON-state period is shortened when the ambient brightness is low to reduce the value of the increased voltage applied to the EL element 206, whereby the luminance of the EL element 206 is controlled.
If the display portion 200 is provided with a plurality of EL elements 206, however, capacitances vary with the EL elements 206 since the conventional EL driving apparatus controls the luminance of the EL element by changing the frequency of the voltage applied to the EL element 206 based on ambient brightness. This varies voltages applied to the individual EL elements 206 and causes undesired variations in the luminance of the whole display portion 200.
In the case where any of the EL elements 206 is replaced with another or where any of the EL elements 206 is electrically disconnected for some reason or other, the capacitance of the EL element 206 is lost, though it is overwhelmingly larger than the capacitance of each of the transistors composing each of the switch elements. Since capacitance is inversely proportional to voltage, the capacitance of the display portion 200 that has been minimized increases the drain voltage of the transistor to a value exceeding the withstand voltage of the transistor, thus incurring the destruction of the transistor.
In the case where the waveform of the voltage applied to the EL element 206 during discharging has a steep falling-edge portion from the increased voltage to 0 V, if the frequency of the pulse signal C2 for controlling the cycle of charging and discharging is on the order of 400 Hz in the audible range, the steep falling-edge portion gives vibrations to the EL element 206 and causes noise. The suppression of the vibrating sound of the EL element is particularly desired in a mobile communication device such as a portable telephone and other electronic devices, which are seriously affected by noise occurring in service. Therefore, the improvement of the waveform of the voltage applied to the EL element 206 is essential to these devices.